When fitting new stems to watches I use these tools :-
Digital calipers,
Fine grade diamond lap
Wire cutter
Pin vice
Now for fitting.
(1) Hold the stem in the pin vice and screw on the crown tightly by hand.
(2) With the movement fitted correctly in the case, insert the stem until it locks in place. Now measure the gap between the case and the underside of the stem. In this case it is 2.16mm.
(3) Subtract 0.2mm from this size and this will give t